Place onion slices on grill or grill pan. Cook 15 minutes, turning once very carefully, until onions are tender and charred on the outside.

4

Cut each roll in half horizontally. Top bottoms of rolls with roast beef and 1 piece cheese. Place onions on top of cheese. Spread mustard on cut side of roll tops. Top sandwiches with roll tops. If desired, place rolls back on cookie sheet, and bake 5 minutes to melt cheese and warm roast beef.

Expert Tips

Use shaved deli turkey or ham in place of the roast beef to mix things up.

Not a fan of spicy mustard? Swap out the spicy mustard for regular yellow mustard or mayonnaise.